I'm going to go out on a limb here and say it had little to do with the sex most likely.
I'd say a bit of buyers remorse coupled with what I'm guessing was a sudden shift to neediness. I'm guessing by the fact that you had a ONS with her that you were not acting like a needy puppy dog that night... and you probably didnt think that you were after and just want to blame it on a poor sexual outing... thats the easy way out, but ultimately if we just blame it on something that I would say if one of the least likely elements of those listed in your post (though interestingly enough the most obvious due to your wording) then you aren't making progress.
You said you called her a few times during the week and she seemed distant. 2 parts there lead me to see neediness...
First the fact that you percieved her as distant means that you wanted the interactions to be "closer" more intimate/friendly/warm/etc then she wanted them to be hence she seems distant yes... but the reason she seemed distant to you was because of her level of "closeness" to you in relation to your desired level of "closeness" with her. If you 2 were equally cold,warm,friendly or unfriendly then neither would seem distant you'd just "be".
Secondly when the disparity of closeness is there, you can really react in 2 ways... give them space, or don't. She showed distance, you kept calling... this causes more distance because she didnt get the space she wanted anyway and eventually leads her to forcing permanent distance which in this case came in the way of the friend zone.
Also not related to your calling, but the way you wrote the post suggests to me that neediness is the issue also. You wrote it in a way that would lead people to blame it on a poor sexual outing... yet if that's what it was, would you really need the validation of other people telling you that's what it was? I don't think so... I think that on some level you believe it was something you projected mentally or emotionally, and are enacting a defense mechanism to saddle the blame on to an easily detached physically based problem.
So now that I have pointed the blame in a different direction I have to provide you with some food for thought on how to help with this issue in the future or I'd just seem like a dick  . It's really pretty basic, ask yourself the single most powerful question there is and answer honestly... if you bullshit yourself then nothing can truly help you. That question is simple "why?"... 3 letters that create the English word which represents the most powerful concept known to humanity. So why? Why where you needy? because you wanted to date her? Because you wanted to sleep with her again? Because you really really liked her shoes? The answers to these questions are yours and yours alone... but the key is to delve deeper... when you have your first answer ask yourself why again... after you do this for a little while you'll finally figure out what your real motivation is... and honestly most people never know what their true motivation for anything is... they assume its some superficial motivation, like sex, or attention... but theres a deeper and more important motivation to be understood just 1 more why away... when you begin to understand your core motivations then you can begin to truly understand yourself... and once you do that you'll be amazed at the happiness and success all around you that you couldn't even see before. 
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gotta keep the kegels going...